The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for designing radio frequency (“RF”) pulses for use in parallel transmission MRI.
Although there is a general consensus that temperature is the true relevant radio frequency (“RF”) related safety parameter, tracking specific absorption rate (“SAR”) in MR exams and in RF pulse design has remained the gold standard. The fact that SAR has remained the gold standard can probably be explained by the complexity of bio-heat equation models and the lack of fast temperature mapping methods.
Given the multi-factorial dependence of temperature, SAR computation has been a great simplification for handling safety, which has been supported by experimental evidence. Whereas RF coil technology, static field intensity, and temperature guidelines have evolved throughout the years, SAR thresholds have essentially remained identical.
In a recent study by A. Massire, et al., “Thermal simulations in the human head for high field MRI using parallel transmission,” J Magn Reson Imaging 2012; 35:1312-1321, a series of thermal simulations was conducted on a numerical human head model containing 20 anatomical structures in order to verify the consistency between the SAR and temperature guidelines for parallel transmission (pTx) RF exposures at 7 T. Thousands of different RF scenarios were simulated. In these models, starting from the equilibrium temperature (i.e. the steady-state obtained when there is no RF heat source), the temperature almost never exceeded 38° C. over 30 minutes of RF pulsing as long as 10-g average SAR did not exceed its limit of 10 W/kg.
These simulations, however, relied on the validity of Pennes' bioheat model whose constant blood temperature assumption is currently debated. Motivated also by the lack of direct correspondence between temperature and SAR, the RF pulse design for a time-of-flight (TOF) sequence under strict absolute temperature constraints was investigated numerically, whereby strict SAR constraints were adjusted in a feedback-based manner to fulfill the temperature constraints. Although the IEC guidelines in general involve upper limits for absolute temperature, the latter actually appears quite dependent on daily fluctuations caused by uncontrolled events occurring in the patient's life (fever, physical activity, stress, weather etc). Furthermore, in vivo absolute temperature measurement with MR is still extremely challenging.